does it mean that all items with indexes like ij with i=>2&&j>1 remain available ?

No. The array @all_with_name is cleared. The element at position 1 in the array reference in $all_with_names[2] has been copied to $gilligan_stuff, so it is there. If the thingy copied was a plain scalar, $gilligan_stuff holds a brand new copy of the contents of the array slot $all_with_names[2][1].

But if the entry in that slot was a reference, by copying that to $gilligan_stuff the reference count of the thingy that reference points to was increased, and the reference in $all_with_names[2][1] pointed to the same thingy.

By destroying the array @all_with_names the original data $all_with_names[2][1] was pointing to has not been deleted, although the slot $all_with_names[2][1] has been destroyed - only the pointer has been deleted. The reference count of the underlying data was decreased by deleting the array which held a reference to it, but until that count goes to zero the data is still allocated and accessible (if you have a refrence :-).

HTH,

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